In the semiconductor industry, a phase-change material (PCM) is a material that is capable of quickly changing between a high-resistance (low-conductivity) state and a low-resistance (high-conductivity) state. A commonly used PCM is chalcogenide glass, which generally refers to inorganic polymers of sulfur, selenium and tellurium that are able to change between an amorphous high-resistance state and a crystalline low-resistance state. Although many chalcogenide glass materials have been studied, most current practical PCM-based IC devices use an alloy comprising Germanium-Antimony-Tellurium (GST). Transitions between low resistance crystalline and high resistance amorphous states are performed either by direct heating of the PCM element (i.e., by passing current pulses of different lengths and amplitudes through the chalcogenide glass material), or by indirect heating of the PCM element (i.e., by passing long and short current pulses through heater elements that are adjacent to the chalcogenide glass material). In either case, the chalcogenide glass is transitioned into its amorphous state using relatively short duration heating and quenching cycles, and the chalcogenide glass is transitioned into its crystalline state using relatively long duration heating and cooling cycles.
The use of PCMs in IC devices dates back to the 1960's, when phase-change random-access memory (PRAM) devices were first introduced. PRAMS and other type of phase-change memory are non-volatile random-access memory (NVM) devices that utilize PCM elements to store data. Each PCM-based NVM cell typically includes a PCM element connected between two electrodes in either a vertical via-type configuration or a horizontal configuration (e.g., with the electrodes formed in the same plane as a chalcogenide film). Data is written/stored by way of causing the PCM element to enter a SET (low resistance crystalline) state or a RESET (high resistance amorphous) state, whereby the data could be subsequently read, e.g., by measuring the resistance of the PCM element.
More recently, PCM-based RF switches were introduced that take advantage of the high OFF/ON resistance ratios exhibited by certain chalcogenide films. Compared with NVM, RF switches require a higher ratio of OFF/ON resistances and lower ON state resistances, and it was recognized that certain GeTe chalcogenide films provided up to 106 OFF/ON resistance ratios (see, e.g., S. Raoux, et al. “Crystallization characteristics of Ge—Sb and Ge—Te phase change materials,” EPCOS, pp. 91-98, September 2009; Y. Shim at. al “RF switches using phase change materials “IEEE MEMS 2013; K. Chua “Development of Phase Change Switches with Low Resistance in the ON state”, PhD Thesis Carnegie Mellon University, PA, 2011; N. Hinnawy, et. al. “Improvements in GeTe-Based Inline Phase-Change Switch Technology for RF Switching Applications”, Compound Semiconductor MANTECH 2014; J. Moon, “11 THz Figure-of-Merit Phase-change RF Switches for Reconfigurable Wireless Front-ends”, IMS 2015). In PCM-based RF switch parlance, OFF state refers to the disordered high-resistance amorphous phase corresponding to the RESET state in NVM terminology, and the ON state refers to the ordered low-resistance crystalline phase corresponding to the SET state in NVM terminology. PCM-based RF switches are transitioned into the OFF state using relatively short heating and cooling cycles, and can be brought back into the ON state by way of longer heating and slower cooling cycles (i.e., to facilitate crystallization). Similar to the case of PCM-based NVM cells, heating of the PCM material in a PCM-based RF switch can be achieved using external heaters, or by way of forcing currents through the chalcogenide glass.
PCM-based RF switches are taught in various patent publications. U.S. Pat. No. 9,368,720 teaches a switch with single phase change material element connected between the switch's input and output ports, a heater, and a thermal dielectric layer (e.g., AlN, diamond or SiC) in between the heater and the phase change material. U.S. Pat. No. 9,293,699 teaches a device similar to that of U.S. Pat. No. 9,368,720, but switching is implemented using a direct current sent through the phase change material (i.e., instead of using a separate heater). PCT publication WO2016028362 also describes another direct heating switch including two electrodes used for RF switch channel operations (i.e., to pass the RF signal), and two additional electrodes for direct heating. The heating element electrodes are configured to draw a current through the same phase change element in a direction transverse to the conductive path used by the RF switch channel. U.S. Pat. No. 9,362,492 teaches integrated phase change switches, including switches connected in series for antenna applications. Auxiliary CMOS circuitry associated with the RF switches is integrated together with phase change switches in the back-end of a CMOS process flow. It is noted that the CMOS transistors serve only as elements of the control circuits, and do not form part of the RF signal paths (i.e., the RF signals only pass through the phase change switches, not through any CMOS transistors).
A problem with the use of the conventional PCM-based RF switches set forth above in antenna applications is that the high power generated across the RF switches can result in undesirable switching when direct-heating-type PCM cells are used. That is, the voltage generated across an OFF-state RF switch branch between the antenna and a transceiver circuit can exceed 50V, especially in cases of impedance mismatch. For example, for the input power of 38 dBm and vertical standing wave ratio (VSWR) of 5:1, the off state antenna branches may be subject to 55V. Because RF switches using direct-heating-type GeTe PCM cells feature transition between the OFF state and the ON state at voltages in the range of 1V to 5V, and at currents on the order of milliamperes, such high voltages generated across an OFF state RF switch can result in undesirable switching of the RF switch to the ON state, rendering the RF switch inoperable.
To avoid the undesirable OFF-to-ON switching problem in RF switches using direct-heating-type PCM cells, a PCM-only RF switch must include multiple PCM cells connected in stacked arrangement (i.e., connected in series) in order to keep the effective voltage across each PCM cell below the PCM cells' threshold/switching voltage. This series-connected PCM cell approach effectively distributes large voltages evenly across the PCM cells in a manner similar to that utilized to prevent damage to MOS transistors in CMOS SOI switches. However, to implement this approach in antenna branch capable of withstanding 55V, dozens of PCM cells must be connected in series. For example, if an RF switch was fabricated using PCM cells having 3V threshold voltages, about twenty connected-in-series PCM cells would be required to keep the voltage across each PCM cell below 3V. Moreover, because the switch's ON-resistance RON is proportional to the number of series-connected PCM cells, and assuming the ON-resistance of a single PCM element on the order of 1 Ohm, additional PCM cells would also have to be connected in parallel with the series-connected chain to reduce the effective ON-resistance of the chain. That is, the only practical way to form RF switches using only direct-heating-type PCM cells is to form arrays of series-and-parallel-connected PCM cells. However, tens of PCMs would be needed to be connected in parallel with the twenty or more connected-in-series PCM cells in order to keep insertion losses at the level of RON produced by a comparable CMOS SOI RF switch (i.e., a CMOS SOI RF switch capable of withstanding 55V). Accordingly, there is little incentive for replacing a relatively high voltage (i.e., above about 15V) CMOS SOI RF switch with a PCM-only RF switch.
Another problem is associated with the distinguishing property of direct-heating-type PCM cells called “threshold switching. Without low (at the level of several volts) thresholds, very high voltages would be required to switch the chalcogenide films from high resistance OFF state to the low resistance ON state. The resistance of the OFF state exponentially decreases when the threshold voltage is approached. Switching is a two-stage process: at the first stage, conductivity is increased (i.e., due to Pool-Frenkel effect, carrier multiplication, intermediate phase change, etc.), causing the active component of the amorphous phase impedance to decrease. The drop of the PCM cell's active resistance leads to high currents (second stage of switching), and then heating results in chalcogenide melting. Compared with MOS transistors entering the punch-through regime, the situation with PCM cells is more dangerous: while avalanche in MOS device may be reversible (i.e., linearity of the switch may be degraded for some time, but then recover), switching of a PCM cell is irreversible and leads to switching of other PCMs in the antenna branch. The typical way to preclude this situation is to include additional PCM cells in the chain. However, with large numbers of PCM cells in a branch, very high “on chip” voltages are required for PCM switching. If voltages for switching are applied to individual PCM cells, leakage passes for RF signal are introduced, although decoupling resistors may be used to limit the values of current that could be forced through the PCM cells.
Although some of the problems associated with direct-heating-type PCM cells mentioned above may be avoided by utilizing indirect-heating-type PCM cells, this alternative approach generates other problems. For example, in RF switches using indirect-heating-type PCM cells, typical currents required to effect switching of the PCM cells are on the order of 20-50 mA (about 10 time higher than for direct switching). This high current demand requires high power drivers, which requires special (non-standard) metallization. Also, using external heaters is expensive, since assumes additional process steps, in particular integration of heat conductive dielectrics.
What is needed is an IC switch that avoids the problems set forth above. In particular, what is needed is a CMOS SOI RF switch that takes advantage of the beneficial qualities associated with the use of PCM cells (i.e., high OFF/ON resistance ratios and low ON resistance) without incurring the problems encountered by conventional PCM-only approaches.